1. Field of the Invention
The present invention relates to an optical amplifier, an optical transmission equipment, an optical transmission system, and a method thereof and in particular, to an optical amplifier, an optical transmission equipment, an optical transmission system, and a method thereof, with which can be obtained a low noise figure (N/F) as well as dispersion compensation for an optical signal therein.
2. Description of Prior Art
In general, it is already known that an input loss of an optical signal at a stage in front of a doped fiber causes deterioration in a S/N ratio thereof, in particular in an optical amplifier using such the doped fiber therein. However, as is described in "Optical Amplifier and Applications thereof" (published by Ohm Co. Ltd., May, 1992) 5-3[1], it is indispensable to insert an optical isolator at the front stage of the doped fiber in the conventional optical transmission equipment, for the purpose of suppressing the reflection of stimulated or induced emission (i.e., ASE: amplified spontaneous emission) light. In the optical transmission equipment with such the construction thereof, however, optical parts necessitated to be inserted at the front stage of the doped fiber are not only the optical isolator. Namely, in general, they also includes optical parts, including a wavelength divider for wavelength of a supervising light, a coupler for monitoring strength of a transmission signal, a multiplexer for multiplexing an pumping or stimulation light, etc., and they have respective losses therein. For instance, for obtaining a gain from 25 dB to 35 dB, it is necessary to combine a semiconductor laser of about 100 mW for excitation and a doped fiber of length from 20 m to 30 m, and in that case, noise figure (i.e., noise index; being abbreviated as NF hereinafter) of the doped fiber cannot be neglected.
In the optical transmission equipment with such the construction, the optical signal which is once damaged or receive losses therein on a transmission path or line is amplified by using the doped fiber having high NF, after being further damaged or lost thereon, therefore it is difficult to keep the NF less than 6 dB, which can be defined by a ratio between the S/N ratio of an input side and that of an output side.
Further, in a case where an optical signal of high velocity is applied onto an optical path of ordinal transmission fiber (NDSF: Non-Dispersion Shifted Fiber), there is a necessity of inserting a device for compensating the dispersion. As the result of this, there is caused another necessity for compensating the loss due to that device for the dispersion compensating.
An example of the structure of such the optical amplifier of conventional art is disclosed in a publication, "Trial of 2.times.2 Bi-directional Relay Optical Fiber Amplifier (BDLA)" (1997 Society Conference of Electronic Information Communication Society, B-10-184), with which the NF can be suppressed at 7.5 dB. Furthermore, a structural example is disclosed for example in U.S. Pat. No. 5,831,754 (Japanese Patent Laying-Open No. Hei 7-301831 (1995)), for compensating the loss due to the disperse compensating device.
In case of transmitting the optical signal by multiple relays or repeaters with use of k optical amplifiers, the deterioration amount in the S/N ratio rises up in proportional to the number of the stages k. Therefore, in an actual optical transmission system where there is an upper limit in total amount of the S/N ratio deterioration, the number of the relay or repeater stages must be lessened following the increase in the S/N deterioration amount of the optical amplifier. As a result of this, the distance of the optical transmission must be shortened.
For instance, under regulation of total amount of the S/N deterioration ratio to be equal to or less than 12 dB, if an optical amplifier of S/N ratio deterioration at 4 dB and an optical amplifier of S/N ratio deterioration at 6 dB are positioned at a distance 80 km, respectively, then the total S/N ratio deterioration amount comes to be 12 dB for the transmission path relayed or amplified with three (3) stages of the optical amplifiers of 4 dB, while the same total S/N ratio deterioration amount of 12 dB is obtained by relaying with two (2) stages of the optical amplifiers of 6 dB. Namely, the optical signal can be transmitted at the distance 240 km with the optical amplifiers of 4 dB in S/N ratio deterioration since it can be relayed at three (3) stages therewith, while it can be transmitted only at the distance 160 km with the optical amplifiers of 6 dB since it can be relayed at only two (2) stages therewith.
Though the S/N deterioration amount is not one being corresponding to the NF one by one, however, it becomes large when the optical amplifier inferior in the NF is applied to, therefore there is a problem that a distance for regenerative relaying or repeating comes to be short, in which the optical signal is returned once into an electric signal to be relayed or repeated.
Furthermore, with the optical amplifier in which the doped fiber is divided into a plurality of stages, a plurality of exciting or pumping light sources are necessary, therefore bringing about a rise-up of cost of the optical amplifier, as well as the large-sizing and increase in electric power consumption thereof.